seed pod
薹 centers on the seed pod of a plant, especially the stalk that forms when a vegetable bolts and produces seeds.
薹 combines the plant radical 艹 with 臺, which likely provides the sound. The character refers to the seed stalk of plants like rapeseed.
The grass top 艹 sits above 臺, which looks like a tall platform. Imagine a tall stalk rising from the grass, topped with a seed pod.
For タイ, picture tying a seed pod to the tall stalk: tie -> タイ, and the pod is secured to the plant.